The present invention relates to a display apparatus for displaying a video image which can operate at plural horizontal scanning frequencies, and more particularly to a so-called multi-scan horizontal deflection frequency controlling device for an automatic horizontal synchronization with a video signal input having a variety of frequencies in a display apparatus.
In a so-called multi-scan type display monitor, it is necessary to obtain stable horizontal and vertical generation output in response to horizontal and vertical synchronous signals having different frequencies. In this case, with respect to the vertical synchronous signal, it is relatively easy to follow its frequency change. However, the frequency of the horizontal synchronous signal changes in the range of several tens of kHz. Therefore., in the case where video signals having different horizontal scanning frequencies are inputted, unless the internal horizontal generation frequency is close to the frequency of the input horizontal synchronous signal, it is impossible to lock a PLL circuit for the internal oscillation.
In such a conventional monitor, in general, a so-called micro-computer is used to count a frequency of an input horizontal synchronous signal H.SYNC and to change data of a digital-to-analog convertor DAC for setting a free running frequency of a horizontal scanning frequency generating circuit in order to effect the synchronization even if the input video signals are switched over. For this reason, a ROM which can set, in advance, horizontal scanning frequencies suitable for the input video signals of various frequencies is provided for storing therein data. The data preset in the ROM are selected and transferred to the DAC to set a free running frequency of the horizontal scanning frequency generating circuit whereby the horizontal synchronous signal H.SYNC is controlled to be close to a horizontal driving signal HD which is an output of the horizontal scanning frequency generating circuit.
However, according to this method, it is necessary to hold, in the ROM in advance, data for outputting HDs having the frequencies corresponding to H.SYNCs of the various video signals. In the case where H.SYNCs which are not preset in the ROM are inputted, it would be impossible to respond to the horizontal synchronous signals.
Also, since the conventional system has no function to measure the frequency of the HD signal, if the data of DAC are offset, a horizontal position on the pictorial image would be offset from a desired position even if it would be possible to effect the lock for the horizontal synchronization.
Furthermore, the micro-computer changes the free running frequencies of the horizontal scanning frequency generating circuit in an open loop control on the basis of the data set in the DAC. However, since the horizontal scanning frequency generating output, i.e., HD is not fed back to the micro-computer, the non-uniformity in characteristics of the components will cause the free running frequencies to have errors. In order to avoid this, it is necessary to perform adjustment of the free running frequencies in the manufacture stage with an adjustment volume or the like. This operation is troublesome.